Ph-responsive lipid derivative

A pH-responsive lipid derivative represented by a structure represented by the following formula (i):

• wherein
• R1 and R2 are each independently an alkyl group having 8 to 24 carbon atoms, or an acyl group having 8 to 24 carbon atoms, or a sterol residue,
• M is a trivalent group represented by >N—CH2—CH2— or an optionally branched hydrocarbon trivalent group having 3 to 7 carbon atoms,
• B is a hydrogen atom, an alkali metal, or an ammonium group,
• X is a linking group containing a bonding group formed by a nucleophilic substitution reaction, a Michael addition reaction, a thiol-ene reaction, or a cyclization addition reaction,
• A is a carboxy group or a sulfo group,
• a1, a2, b, c, d, and e are each independently 0 or 1,
• f is an integer of 0 to 5,
• h is an integer of 1 to 3,
• g and n are each independently 1 or 2,
• m is 5 to 300, and
• Ra is H, —COCH3, or —CO—(CH2)b1—COOH (wherein b1 is 2 or 3).